1. Field of the Invention
The present invention relates to a display apparatus having a plurality of display panels and accompanying drive control method.
2. Description of the Related Art
As the display panel in portable electronic apparatus, such as cellular phones, Personal Digital Assistant (PDAs), etc., a lot of display apparatuses have a liquid crystal display (hereinafter, denoted as “LCD”) panel with thin shape, light weight and low power consumption. In particular, many display apparatuses have a LCD panel of the active-matrix method which uses thin film transistors (TFTs) as the display panel.
On the LCD panel of such display apparatuses, a plurality of scanning lines and a plurality of signal lines are arranged to mutually intersect and display pixels are formed near each intersection point.
FIG. 9 is an equivalent circuit view showing an example configuration of a display pixel.
As seen in FIG. 9, each display pixel composes a thin film transistor (TFT) 91 connected to a scanning line G and a signal line S, a pixel electrode 92 connected to the signal line S via the TFT 91, a common electrode 93 arranged in a location which faces toward the pixel electrode 92, a pixel capacitance 94 with liquid crystal filled between the pixel electrode 92 and the common electrode 93, and an auxiliary capacitance 31 connected in parallel to the pixel capacitance 94 which holds the applied voltage of the pixel capacitance 94. An image display is actualized according to the arrangement of the liquid crystal molecules which change by an electric field formed between the pixel electrode 92 and the common electrode 93.
In particular, the TFT 91 gate electrode is connected to the scanning line G, the source electrode is connected to the signal line S, and the drain electrode is connected to the pixel electrode 92 of the pixel capacitance 94 and one electrode of the auxiliary capacitance 31. Also, a predetermined common voltage VCOM (common electrode signal) is applied to the common electrode 93. The other electrode of the auxiliary capacitance 31 is connected to a common lines C (auxiliary capacitance lines)•• and the predetermined common voltage VCOM (common electrode signal) is applied. When high electric potential is applied to the TFT 91 gate terminal via the scanning line G, the TFT 91 switches to an “ON” state. An electric field is formed between the pixel electrode 92 and the common electrode 93 by the electric potential of the signal line S being applied to the pixel electrode 92 and drives the liquid crystal filled between such electrodes.
Furthermore, in order to visually capture a display image in an LCD, backlight is provided, for example, by an LED in the rear surface of the LCD panel. The permeated amount of light emitted from the backlight is controlled by the arrangement of the liquid crystal molecules. The luminosity of each display pixel is adjusted and the desired image is displayed.
Moreover, in recent years, an electronic apparatus equipped with a display apparatus having a plurality of LCD panels is known as represented by a foldable type cellular phone provided with a main screen and a sub-screen. In a display apparatus having such a plurality of LCD panels, in order to simplify the structure, the signal lines for each LCD panel are connected together. For example, when having two LCD panels constituting the main screen and the sub-screen, the wiring of the signal lines allocated in the main LCD panel is extended to the sub-LCD panel, which is used to drive both LCD panels with one source driver. Also, the scanning lines are wired separately for each LCD panel. The common voltage VCOM applied to the common electrode of each LCD panel is generated and applied to each LCD panel.
FIG. 10 is a diagram showing a signal waveform example of a conventional prior art case with the signal lines connected together and performing display drive of the two LCD panels for the main screen and the sub-screen with common signal lines.
In FIG. 10, the number of scanning lines of the main LCD panel is denoted as “m1” and the number of scanning lines of the sub-LCD panel is denoted as “m2.” As shown in this diagram, the time axis is set on the horizontal axis. The sequence listed from the top shows the polarity of the display signal applied to the signal lines S, the common voltage VCOM1 applied to the common electrode of the main LCD panel, the common voltage VCOM2 applied to the common electrode of the sub-LCD panel and a signal waveform of each scanning line G of the two LCD panels.
As seen in this diagram, in the case of having two LCD panels, a 1 frame period constitutes a back portion (BP), a main screen display period which makes the main LCD panel the displaying object, a middle portion (MP), a sub-screen display period which makes the sub-LCD panel the displaying object and a front portion (FP). BP is the period from ending output of a horizontal synchronization signal until beginning output of the display signal for the main LCD panel. MP is the period from ending output of the display signal for the main LCD panel until beginning output of the display signal for the sub-LCD panel. FP is the period from ending output of the display signal for the sub-LCD panel until beginning output of the horizontal synchronization signal and is a non-display period known as a retrace line period. Here, “1 frame period” indicates a period which displays one image on each LCD panel.
In the main screen display period, as the scanning lines G1˜Gm1 of the main LCD panel are sequentially scanned and set in a selective state, the display signal of the image to be displayed on that LCD panel is applied to the signal lines S. Also, in the sub-screen display period, as the scanning lines Gm1+1˜Gm1+m2 of the sub-LCD panel are set in a selective state, the display signal of the image to be displayed on that LCD panel is applied to the signal lines S. Namely, in 1 frame period, the desired image is displayed on each LCD panel by sequentially making the two LCD panels the displaying object.
Generally, in an LCD, reversal drive is performed which reverses the polarity of the electric field in predetermined cycles between the pixel electrode and common electrode filled with liquid crystal. In an LCD as mentioned above, the arrangement of the liquid crystal molecules is determined corresponding to the electric field between electrodes. However, when direct current is applied between these electrodes, it can induce overheating and seizing which causes the liquid crystal to deteriorate and ultimately component failure will occur. Accordingly, this is prevented by cyclically reversing the polarity of the electric field between electrodes.
As reversal drive methods, line reversal drive and frame reversal drive are commonly implemented. Line reversal drive is a method which reverses each frame period while reversing the polarity of each display pixel in every scanning line. Moreover, frame reversal drive is a method which reverses the polarity for each display pixel in every frame period.
More specifically, the signal waveform shown in FIG. 10 can be assumed as line reversal drive and frame reversal drive as the polarity of the display signal and the common voltage VCOM1, VCOM2 are reversed in every scanning line and reversed in every frame period. The common voltage VCOM1, VCOM2 to the two LCD panels is controlled to always become the same polarity (corresponding polarity) and the display signal is controlled so that the common voltage VCOM1, VCOM2 and the polarity become reversed.
Apart from that, in a foldable type of cellular phone provided with two LCD panels for the above-mentioned main screen and sub-screen, generally the sub-LCD panel is placed on the back surface side of the main LCD panel. Also, many of these apparatus are kept in a folded position so as that the main LCD panel is on the inner side. When in a folded position in order to reduce power consumption, the main LCD panel is set in the non-display state. Meanwhile, viewing by the user is accomplished in the sub-LCD panel located on the outer side of the cellular phone as the sub-LCD panel is set in the display state. Conversely, when such a foldable type cellular phone is in an opened position, viewing by the user is accomplished in the main LCD panel as the main LCD panel is set in the display state and the sub-LCD panel is set in the non-display state.
Thus, in an LCD equipped with two LCD panels constituting the main screen and the sub-screen, for example, when the main LCD panel is set in the non-display state, this LCD panel is normally white and in this case ordinarily set in a white display state. Each scanning line of the main LCD panel is set in the non-display state which is a non-scan state. As for the sub-LCD panel, each scanning line is sequentially scanned and the screen display is performed corresponding to the display signal applied to the signal lines. However, even if the scanning lines of the main LCD panel are set in the non-scan state where the TFT is switched “OFF,” leakage current occurs between the TFT source and drain. Further, since the signal lines are mutually arranged in the two LCD panels, the electric potential of the signal lines changes with the display signal to the sub-LCD panel. For this reason, even if the main LCD panel is set in the non-display state wherein the scanning lines are in the non-scan state, the electric field applied to the liquid crystal changes due to this leakage current and the non-display state is not satisfactorily sustainable.
Therefore, while setting the main LCD panel in the non-display state (for example, white display state) as the display signal of a white display is applied to each signal line of this LCD panel at predetermined timing, an operation is periodically performed which maintains the entire surface of this LCD panel in a white display state by scanning each scanning line. This operation is called a “refresh operation” and required to be performed at the rate of one time in a plurality of frames.
FIG. 11 is a diagram showing a signal waveform example of a conventional prior art in the case of two LCD panels with common signal lines, the main LCD panel in the non-display state and the sub-LCD panel in the display state, and a refresh operation of the main LCD panel performed at the rate of one time every three frames.
As shown in FIG. 11, the time axis is set on the horizontal axis. The sequence listed from the top shows a signal waveform of the gate numbers to which the scanning signal is applied, the polarity of the display signal applied to the signal lines S, the common voltage VCOM1 applied to the common electrode of the main LCD panel and the common voltage VCOM2 applied to the common electrode of the sub-LCD panel.
For example, at the 3n+1 frame in which a refresh operation of the main LCD panel 11 is performed in the main screen display period (main screen refresh operation period), the scanning lines G1˜Gm1 of the main LCD panel are sequentially scanned and the display signal for setting this LCD panel as a full screen white display is applied to the signal lines S. Also, in the sub-screen display period, the scanning lines Gm1+1˜Gm1+m2 of the sub-LCD panel are sequentially scanned and the display signal of the image to be displayed on this LCD panel is applied to the signal lines S.
Next, the 3n+2 frame and 3(n+1) frame, in the main screen display period, the scanning lines of the main LCD panel set in the non-scan state and the display signal is not applied to the signal lines S. Also, in the sub-screen display period, the scanning lines Gm1+1˜Gm1+m2 of the sub-LCD panel are sequentially scanned and the display signal of the image to be displayed is applied to the signal lines S.
Subsequently, the 3(n+1)+1 frame, in the main screen display period a refresh operation is performed again and an operation for displaying an image on the sub-LCD panel in the sub-screen display period is performed. In this manner, for example, by performing a refresh operation of an LCD panel set in non-display at the rate of one time every three frames, the non-display state of this LCD panel is sustained.
Apart from that, in FIG. 11, although reversal drive of the polarity for the common voltage VCOM2 of the sub-LCD panel is performed in every line, reversal drive of the polarity for the common voltage VCOM1 of the main LCD panel is performed every three frames. In this case, in a frame (for example, 3n+2 frame, etc.) in which a refresh operation of the main LCD panel is not performed, the period corresponding to the main screen display period exists unchanged. In this period, reversal drive of the polarity for each line of the common voltage VCOM2 of the sub-LCD panel is continued without interruption. Because of this, when the main LCD panel is set in a non-scan state, power consumption related to the reversal drive for the common voltage VCOM2 of the sub-LCD panel becomes a waste. Accordingly, in a display apparatus having two LCD panels, even if it is the case where one of the LCD panels has been set in the non-display state, wasteful power consumption occurs related to the drive of the LCD panel side set in the non-display state.